docs(changelog): track known backend download/metadata bugs

Duplicate detection (filename-only), missing download size cap (52MB wrong
video), and uncaptured iTunes album artist (collectionArtistName) affecting
Navidrome grouping/sorting.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -224,4 +224,13 @@ Versioning: two-part `MAJOR.MINOR` — each release bumps the minor (features an
 - **Web + Android** — yt-dlp backend update / maintenance
 
 ### Known Bugs
-_(none currently tracked)_
+- **Backend (download)** — Duplicate detection is filename-only and exact: a track re-downloads
+  when the cleaned filename changes (e.g. `… - recognize - Single.mp3` vs `… - recognize.mp3`),
+  leaving two copies. Should match on normalized artist+title (ignore release suffix / variant /
+  track prefix), not the raw filename.
+- **Backend (download)** — No size/length safety cap: a wrong long video can be saved as a huge
+  file (observed a 52 MB mp3 ≈ 28 min vs the correct 4 MB ~3 min). Needs a `--max-filesize` /
+  duration cap as defense-in-depth.
+- **Backend (metadata/sorting)** — iTunes `collectionArtistName` (album artist) is not captured,
+  so `album_artist` can be wrong on compilations / various-artists releases, splitting albums and
+  mis-sorting them in Navidrome.